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Generally, it is found that the current i in the ground generates the soil composition alone are too high. It is, however, possible
changes in the material. to define potential shift areas for natural soils.
But a change to the material also means that the voltage of
the individual metals changes with respect to the ground. This A polarisation below +20 mV is generally not hazardous. Po-
potential drift caused by the corrosion current i is called polari- tential shifts exceeding +100 mV are definitely hazardous. Be-
sation. The strength of the polarisation is directly proportional tween 20 and 100 mV there will always be cases where the
to the current density. Polarisation phenomena now occur at polarisation causes considerable corrosion effects.
the negative and positive electrode. However, the current den- To sum up: The presence of metal and electrolytic anodes and
sities at both electrodes are mostly different. cathodes connected so as to be conductive is always a pre-
requisite for the formation of corrosion cells (galvanic cells).
To illustrate this, the following example is considered: Anodes and cathodes consist of:
A well-insulated steel gas pipe in the ground is connected to ¨ Materials
copper earth electrodes. – Different metals or different surface quality of a metal
If the insulated pipe has only a few small defects, there is a (contact corrosion)
high current density at these points resulting in quick corrosion – Different structural constituents (selective or intercrystal-
of the steel. In contrast, the current density is low in case of a line corrosion)
much larger area of the copper earth electrodes via which the ¨ Electrolytes
current enters. Thus, the polarisation is greater in case of the – Different concentration (e.g. salt content, ventilation)
more negative insulated steel pipe than in case of the posi-
tive copper earth electrodes. The potential of the steel pipe is In corrosion cells, the anodic areas always have a more nega-
shifted to more positive values. Thus, the potential difference tive metal / electrolyte potential than the cathodic areas.
between the electrodes also decreases. The magnitude of the The metal / electrolyte potentials are measured using a satu-
corrosion current therefore also depends on the polarisation rated copper sulphate electrode mounted in the immediate vi-
characteristics of the electrodes. cinity of the metal in or on the ground. If there is a conductive
The strength of the polarisation can be estimated by measur- metal connection between the anode and cathode, the poten-
ing the electrode potentials in case of a disconnected circuit tial difference causes a direct current in the electrolyte which
which avoids the voltage drop in the electrolyte. Recording in- passes from the anode into the electrolyte by dissolving metal
struments are usually used for such measurements since there before entering the cathode again.
is frequently a rapid depolarisation immediately after the cor-
rosion current is interrupted. The “area rule” is often used to estimate the average anodic
If a high polarisation is now measured at the anode (the more current density J A :
negative electrode), i.e. if there is an obvious shift to more pos-
itive potentials, there is a high risk that the anode will corrode. J = U C U A A C in A/m 2
Let us now return to the corrosion cell steel (black) in con- A A
crete / steel, galvanised in sand (Figure 5.5.7.2.4). With respect C A
to a distant copper sulphate electrode, it is possible to meas-
ure a potential of the interconnected cell between –200 mV J A Average anodic current density
and –800 mV depending on the ratio between the anode and U A , U C Anode or cathode potentials in V
cathode and the polarisability of the electrodes. ϕ C Polarisation resistivity of the cathode in Ωm 2
If, for example, the area of the reinforced concrete foundation A A , A C Anode or cathode surfaces in m 2
is very large compared to the surface of the galvanised steel
wire, a high anodic current density occurs at the latter so that The polarisation resistance is the quotient of the polarisation
it is polarised to almost the potential of the reinforcing steel voltage and the total current of a mixed electrode (an elec-
and destroyed in a relatively short time. trode where more than one electrode reaction takes place).
Consequently, a high positive polarisation always indicates an In practice, it is possible to determine the driving cell voltage
increased risk of corrosion. U C – U A and the areas A C and A A as an approximation in order
In practice, it is of course important to know the limit above to estimate the corrosion rate, however, the values of ϕ A (po-
which a positive potential shift denotes an imminent risk of larisation resistivity of the anode) and ϕ C are not available with
corrosion. Unfortunately, it is not possible to give an exact sufficient accuracy. They depend on the electrode materials, the
value for this which applies in every case since the effects of electrolytes and the anodic and cathodic current densities.

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